Hydrogen absorbing alloy powder and process for producing same

ABSTRACT

The invention provides a hydrogen absorbing alloy electrode obtained by the step P 1  of preparing a hydrogen absorbing alloy powder containing cobalt and nickel, the step P 2  of subjecting the surfaces of the alloy particles to a reduction treatment with high-temperature hydrogen by holding the powder in a high-temperature hydrogen atmosphere under the conditions of temperature, pressure and time sufficient to reduce oxides formed in a surface layer portion of each of the alloy particles, not melting the alloy particles and not permitting the alloy particles to absorb hydrogen, the step P 3  of treating the resulting powder with an acid or alkali by immersing the powder in an acid or alkaline aqueous solution, followed by suction filtration, washing with water and drying, and the step P 4  of applying the resulting power to an electrically conductive substrate and shaping the substrate in the form of the electrode. The electrode thus provided has higher activity than conventionally.

TECHNICAL FIELD

[0001] The present invention relates to a hydrogen absorbing alloypowder for use as a material for electrodes (negative electrodes) ofmetallic oxide-hydrogen batteries such as nickel-hydrogen batteries, anda process for producing the powder, and more particularly to the surfacetreatment of a hydrogen absorbing alloy powder.

BACKGROUND ART

[0002] Hydrogen absorbing alloy electrodes serving as the negativeelectrodes of nickel-hydrogen batteries are prepared by pulverizing ahydrogen absorbing alloy ingot to obtain a hydrogen absorbing alloypowder, admixing a binder with the powder and shaping the mixture in theform of the electrode. The hydrogen absorbing alloys heretoforedeveloped include those of AB₅-type rare earths having a crystalstructure of the CaCu₅ type, such Mm—Ni alloys, and TiNi₂ alloys havinga Laves-phase structure of the C14-type or C15-type.

[0003] With the nickel-hydrogen batteries having a hydrogen absorbingalloy electrode as the negative electrode, a gas-phase reaction and anelectrochemical reaction proceed at the same time on the surface of thehydrogen absorbing alloy owing to the contact of the alloy surface withan alkaline electrolyte. More specifically, in the relationship betweenthe hydrogen pressure and the temperature, hydrogen is absorbed by thealloy, or the alloy desorbs hydrogen (gas-phase reaction). In thevoltage-current relationship, on the other hand, application of voltage(charging) permits the alloy to absorb the hydrogen produced by theelectrolysis of water, and the delivery of current (discharging)oxidizes hydrogen to form water (electrochemical reaction). Theproperties of the alloy surface are therefore important in improving theperformance of the nickel-hydrogen battery.

[0004] Accordingly, to improve the activity of the hydrogen absorbingalloy for use in nickel-hydrogen batteries, it is conventional practiceto immerse a hydrogen alloy powder in an aqueous acid solution forsurface treatment as disclosed in JP-B-225975/1993, or in an aqueousalkaline solution for surface treatment as disclosed inJP-B-175339/1988. The surface treatment removes an oxide film formed inthe surface layer portions of the alloy particles, permitting rare-earthelements (such as La) to dissolve out and forming a nickel- orcobalt-rich layer in the surfaces layer portions of the particles,whereby the alloy is given improved electrochemical catalytic activity.

[0005] However, we have found that the conventional surface treatmentstill fails to afford sufficient activity although forming the nickel-or cobalt-rich layer in the surface layer portions of the alloyparticles.

[0006] An object of the present invention is to provide a hydrogenabsorbing alloy powder having higher activity than conventionally, aprocess for producing the powder, a hydrogen absorbing alloy electrodewherein the power is used, and a metallic oxide-hydrogen batterycomprising the electrode.

DISCLOSURE OF THE INVENTION

[0007] In producing a hydrogen absorbing alloy powder of the presentinvention, a starting hydrogen absorbing alloy powder containing nickeland cobalt is held in a high-temperature hydrogen atmosphere under theconditions of temperature, pressure and time sufficient to reduce oxidesformed in a surface layer portion of each of the alloy particles 1, notmelting the alloy particles 1 and not permitting the alloy particles toabsorb hydrogen, and thereafter surface-treated with an acid or alkalinetreating liquid. In this process, the temperature is in the range of100° C. to 900° C., the pressure is in the range of 1 atm to 3 atm, andthe time is in the range of 30 minutes to 10 hours. The acid treatingliquid is, for example, a hydrochloric acid solution. The alkalinetreating liquid to be used is at least one aqueous solution selected,for example, from among aqueous solution of KOH, aqueous solution ofNaOH and aqueous solution of LiOH.

[0008] The hydrogen absorbing alloy powder obtained by the aboveproduction process is applied to an electrically conductive substrateand shaped in the form of an electrode to prepare a hydrogen absorbingalloy electrode of the invention.

[0009] The oxide film formed in the surface layer portions of the alloyparticles 1 in the step of preparing the starting hydrogen absorbingalloy powder is reduced by the high-temperature hydrogen atmosphere(reduction treatment with high-temperature hydrogen) of the aboveprocess and thereby converted to a first metal-rich layer 3 which isenriched in metals (nickel and cobalt). Since the temperature, pressureand time for the treatment are adjusted to the respective rangesmentioned, the oxide film is fully reduced without the likelihood of thealloy particles 1 melting or absorbing hydrogen.

[0010] The alloy powder is thereafter subjected to a surface treatmentwith the acid or alkaline treating liquid, whereby oxides of rare-earthelements (such as La), or Al, etc. are allowed to dissolve out from asurface layer portion of the first metal-rich layer 3. A secondmetal-rich layer 4 further enriched in the metals (nickel and cobalt) isformed in the surface layer portion of the first metal-rich layer 3. Thefirst metal-rich layer 3 is internally studded with relatively smallclusters 30 of the metals (nickel and cobalt), while the secondmetal-rich layer 4 is visually found to be internally studded with manyrelatively large clusters 40 of the metals (nickel and cobalt).

[0011] According to the present invention, the first metal-rich layer 3formed by the reduction treatment with high-temperature hydrogen andenriched in nickel and cobalt is treated with an acid or alkali to formthe second metal-rich layer 4 which is further enriched in nickel andcobalt. The invention therefore affords higher activity than the priorart wherein an acid treatment or alkali treatment only is conducted.

[0012] The hydrogen absorbing alloys usable according to the inventionare those having a crystal structure of the CaCu₅ type, and alloyshaving a Laves-phase structure of the C14-type or C15-type. Preferableto use are alloys having a crystal structure of the CaCu₅ type.

[0013] Examples of useful alloys having the CaCu₅-type crystal structureare those represented by MmNi₂CoAlMn and obtained by substituting the Laof LaNi₅ with Mm (misch metal) which is a mixture of rare-earthelements, i.e., alloys represented by the formula MmNi_(x)M1_(y)M2_(z)(wherein Mm is a mixture of rare-earth elements, M1 is at least oneelement selected from among Co, Al and Mn, M2 is a transition metaldifferent from M1, x is a positive real number, x, y and z are such that4.7≦x+y+z≦5.4).

[0014] Examples of useful alloys having a Laves-phase structure arethose represented by AB₂ (wherein A is at least one of Ti and Zr, and Bis at least one element selected from among Ni, Co, V, Mn, Fe and Cr).More specifically, TiNi₂ and Ti_(0.5)Zr_(0.5)Ni₂ are useful.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a scheme showing the process of the invention forproducing a hydrogen absorbing alloy electrode;

[0016]FIG. 2 is a sectional view showing an alkaline battery;

[0017]FIG. 3 is a diagram for illustrating the effect of a reductiontreatment of the invention with high-temperature hydrogen;

[0018]FIG. 4 is a diagram showing the structure of surface layer portionof a particle of hydrogen absorbing alloy embodying the invention;

[0019]FIG. 5 is a table showing the results obtained by analyzing thesurface layer portion of the alloy particle to determine thecompositions thereof;

[0020]FIG. 6 is a perspective view partly broken away and showing theconstruction of a test cell;

[0021]FIG. 7 is a table showing the results of tests conducted tosubstantiate the advantage of the invention; and

[0022]FIG. 8 is another table showing the results of the tests.

BEST MODE OF CARRYING OUT THE INVENTION

[0023]FIG. 2 shows the construction of a nickel-hydrogen battery (forexample, 1000 mAh in battery capacity) of AA size and of the typewherein the positive electrode is dominant, and the present invention isto be practiced. The illustrated battery, which is an alkaline battery,has a closed construction comprising a positive electrode 11, negativeelectrode 12, separator 13, positive electrode lead 14, negativeelectrode lead 15, external positive terminal 16, can 17 serving also asa negative terminal, closure 18, etc. The positive electrode 11 and thenegative electrode 12 are accommodated, as rolled up in a spiral formwith the separator 13 interposed therebetween, in the can 17. Thepositive electrode 11 is connected by the lead 14 to the closure 18, andthe negative electrode 12 by the lead 15 to the can 17. An insulatingpacking 20 is provided at the junction of the can 17 and the closure 18to seal off the battery. A coiled spring 19 is interposed between theexternal positive terminal 16 and the closure 18. The spring 19 iscompressed to release a gas from inside the battery to the atmospherewhen the internal pressure of the battery builds up abnormally.

[0024] A hydrogen absorbing alloy electrode for use as the negativeelectrode 12 is produced by the steps shown in FIG. 1.

[0025] First, a hydrogen absorbing alloy powder is prepared as specifiedin composition and particle size (step P1). For example, Mm, Ni, Co, Aland Mn are mixed together in the mole ratio of 1.0:3.1:1.0:0.3:0.6, andthe mixture is melted in an arc melting furnace having an argonatmosphere and thereafter allowed to cool spontaneously to obtain aningot of hydrogen absorbing alloy represented by the formulaMmNi_(3.1)CoAl_(0.3)Mn_(0.6). The ingot is mechanically pulverized inthe air to obtain a hydrogen absorbing alloy powder having a meanparticle size of 80 micrometers.

[0026] Next, the alloy powder is placed into a heat-resistantpressure-resistant container of stainless steel and heated at 300° C.after evacuation, hydrogen gas is then introduced into the container to1.2 atm, and the powder is held in this state for 30 minutes. In thisway, the alloy particles are subjected to a surface treatment (reductiontreatment with high-temperature hydrogen, step P2).

[0027] The temperature, pressure and time for the reduction treatmentwith high-temperature hydrogen are not limited to the above values butare so determined that the oxide film formed in the surface layerportions of the alloy particles as will be described below can be fullyreduced without permitting the particles to melt and to absorb hydrogen.The treatment is conducted, for example, at a temperature in the rangeof 100° C. to 900° C. and at a pressure in the range of 1.0 atm to 3.0atm for 30 minutes to 10 hours.

[0028] Subsequently, the alloy powder resulting from the reductiontreatment is immersed in an acid aqueous solution, for example, in a0.5N hydrochloric acid solution (room temperature), at a pH of 0.3 for 2hours, followed by suction filtration, washing with water and drying,whereby the powder is subjected to an acid treatment (step P3).

[0029] The acid aqueous solution is not limited to the hydrochloric acidsolution; an aqueous solution having a strong acidity of 0.3 to 2.0 inpH can be used. For example, a sulfuric acid solution or nitric acidsolution is usable. In view of the battery characteristics, thehydrochloric acid solution is more preferable than the sulfuric acidsolution or nitric acid solution in that the aqueous solution is freefrom the sulfate radial (SO₄ ²⁻) or nitrate radical (NO₃ ⁻).

[0030] The hydrogen absorbing alloy powder can be treated with analkaline aqueous solution instead of the acid treatment. The alloypowder is immersed, for example, in a 30 wt. % aqueous solution ofpotassium hydroxide (80° C.), useful as an electrolyte, for 2 hours,followed by suction filtration and drying. The alkaline aqueous solutionis not limited to the aqueous solution of potassium hydroxide but can bea strongly alkaline aqueous solution consisting predominantly ofpotassium hydroxide (KOH), such as an aqueous solution of KOH and LiOH,aqueous solution of KOH and NaOH or aqueous solution of KOH, NaOH andLiOH. An aqueous solution of LiOH and NaOH is also usable.

[0031] The hydrogen absorbing alloy powder resulting from the acidtreatment is thereafter mixed with a 5 wt. % aqueous solution of abinder such as PEO (polyethylene oxide) in the ratio of 100:20 by weightto prepare a paste, which is applied to opposite surfaces of a substrateof punching metal plated with nickel, followed by drying at roomtemperature and cutting to a predetermined size, whereby a hydrogenabsorbing alloy electrode is produced (step P4).

[0032] The electrode thus obtained is incorporated as the negativeelectrode into the nickel-hydrogen battery shown in FIG. 2. A sinterednickel electrode is usable as the positive electrode, analkali-resistant nonwoven fabric as the separator, and a 30 wt. %aqueous solution of potassium hydroxide as the electrolyte.

[0033] In the process shown in FIG. 1 for producing the hydrogenabsorbing alloy electrode, the surfaces of the alloy particles asprepared by step P1 come into contact with the air or the water in theair, with the result that an oxide film 2 of nickel oxide and cobaltoxide is formed in the surface layer portions of the alloy particles 1as shown in FIG. 3, (a).

[0034] The oxide film 2 is thereafter reduced with high-temperaturehydrogen in step P2 and thereby converted to a first metal-rich layer 3which is enriched in nickel and cobalt as shown in FIG. 3, (b).

[0035] Further in step P3, rare-earth elements such as La dissolve outfrom a surface layer portion of the first metal-rich layer 3, with theresult that a second metal-rich layer 4 further enriched in nickel andcobalt is formed in the surface layer portion of the first metal-richlayer 3.

[0036]FIG. 4 is a diagram schematically showing the surface layerportion of the alloy particle obtained by the above process, as observedunder a transmission electron microscope. A sample was prepared by ionetching for the observation of the surface layer portion.

[0037] As illustrated, the first metal-rich layer 3 is internallystudded with relatively small clusters 30 of nickel and cobalt, whilethe second metal-rich layer 4 is internally studded with many relativelylarge clusters 40 of nickel and cobalt. Thus, the second metal-richlayer 4 further enriched in nickel and cobalt is formed in the surfacelayer portion of the first metal-rich layer 3.

[0038]FIG. 5 shows the proportions of component elements (proportion, inatm %, of each component element in the entire composition of the layer)in the first metal-rich layer 3 and the second metal-rich layer 4 of thehydrogen absorbing alloy powder, as determined by energy dispersiveX-ray analysis (EDX) using a field emission scanning transmissionelectron microscope (FESTEM).

[0039] The proportion of nickel (Ni) and the proportion of cobalt (Co)are both greater in the second metal-rich layer 4 than in the firstmetal-rich layer 3. This indicates that the second metal-rich layer 4further enriched in nickel and cobalt is formed in the surface layerportion of the first metal-rich layer 3.

[0040] Thus, the first metal-rich layer 3 formed by the reductiontreatment with high-temperature hydrogen and enriched in nickel andcobalt is treated with an acid to form the second metal-rich layer 4which is further enriched in nickel and cobalt. The surface treatingprocess of the invention therefore gives higher electrochemicalcatalytic activity to the hydrogen absorbing alloy electrode than theprior art wherein an acid treatment only is conducted.

[0041]FIGS. 7 and 8 show the results of tests conducted to substantiatethe advantage of the surface treating process of the present invention.

[0042] Described below are preparation of a test device, test method andtest results.

[0043] (1) Preparation of hydrogen absorbing alloy powders

[0044] Mm (a mixture of rare-earth elements), Ni, Co, Al and Mn(elemental metal with a purity of 99.9%) were mixed together in the moleratio of 1.0:3.1:1.0:0.3:0.6, and the mixture was melted in an arcmelting furnace having an argon atmosphere and thereafter allowed tocool spontaneously to obtain an ingot of hydrogen absorbing alloyrepresented by the formula MmNi_(3.1)CoAl_(0.3)Mn_(0.6). The ingot wasmechanically pulverized in the air to obtain a hydrogen absorbing alloypowder (untreated alloy powder 1) adjusted to a mean particle size of 80micrometers.

[0045] A hydrogen absorbing alloy powder (untreated alloy powder 2) wasalso prepared with the same composition and mean particle size by thegas atomizing process.

[0046] (2) Preparation of alloy powders by reduction

[0047] Untreated alloy powders 1 and 2 were placed into respectiveheat-resistant pressure-resistant containers of stainless steel andheated at varying temperatures of 50° C. to 950° C. after evacuation,hydrogen gas was then introduced into the containers to 1.2 atm, and thepowders were held in this state for 30 minutes to obtain alloy powdersreduced with high-temperature hydrogen.

[0048] (3) Preparation of alloy powers treated with acid

[0049] Untreated alloy powders 1 and 2, and the reduced alloy powderswere each immersed in a 0.5N hydrochloric acid solution (roomtemperature) at a pH of 0.3 for 2 hours, followed by suction filtration,washing with water and drying to obtain acid-treated alloy powders.

[0050] (4) Preparation of alloy powders treated with alkali

[0051] Untreated alloy powders 1 and 2, and the reduced alloy powderswere each immersed in a 30 wt. % aqueous solution of potassium hydroxide(80° C.), which is for use as an electrolyte, for 2 hours, followed bysuction filtration and drying to obtain alkali-treated alloy powders.

[0052] (5) Preparation of alloy electrodes

[0053] A 0.5 g quantity of each of various hydrogen absorbing alloypowders thus prepared was admixed with 0.1 g of PTFE, the mixture wasapplied to an expanded nickel porous body serving as a conductivesubstrate, and the body was then pressed at 1200 kgf/cm² for shaping,whereby a hydrogen absorbing alloy electrode was prepared in the form ofa disk with a diameter of 20 mm.

[0054] (6) Assembly of test cells

[0055] The electrodes thus obtained were used as test electrodes(negative electrodes) to assemble test cells like the one shown in FIG.6.

[0056] As illustrated, the test cell has arranged in an insulatingclosed container 21 of polypropylene a test electrode 22 which is thehydrogen absorbing alloy electrode to be tested, a sintered nickelelectrode 23 in the form of a hollow cylinder and having a sufficientlygreater electrochemical capacity than the test electrode 22, and asintered nickel reference electrode 24 in the form of a plate. Thenickel electrode 23 is supported by the lower end of a positiveelectrode lead 26 connected to the top wall 25 of the closed container21. The test electrode 22 is vertically supported by the lower end of anegative electrode lead 27 connected to the top wall 25 of the container21, and is accommodated inside the nickel electrode 23 centrallythereof.

[0057] The positive electrode lead 26 and the negative electrode lead 27extend through the top wall 25 of the closed container and are exposedto the outside and connected to a positive terminal 28 and a negativeterminal 29, respectively. The test electrode 22 and the sintered nickelelectrode 23 are held immersed in an alkaline electrolyte (30 wt. %aqueous solution of potassium hydroxide). The closed container 21 isfilled with nitrogen gas in a space above the alkaline electrolyte,whereby the test electrode 22 is subjected to a predetermined pressure(5 atm). Connected to the center portion of the top wall 25 of theclosed container 21 is a relief pipe 32 equipped with a pressure gauge30 and a relief valve 31 for preventing the internal pressure of thecontainer 21 from increasing above a predetermined value.

[0058] (7) Assembly of alkaline batteries

[0059] Each hydrogen absorbing alloy powder and a 5 wt. % aqueoussolution of PEO (polyethylene oxide) were mixed together in the ratio of100:20 by weight to prepare a paste, which was applied to oppositesurfaces of punching metal (conductive substrate) plated with nickel,followed by drying at room temperature and cutting to a predeterminedsize, to prepare a hydrogen absorbing alloy electrode. A nickel-hydrogenbattery (1000 mAh in battery capacity) of AA size and of the typewherein the positive electrode is dominant shown in FIG. 2 was thenassembled using the electrode as the negative electrode. A sinterednickel electrode was usable as the positive electrode, analkali-resistant nonwoven fabric as the separator, and a 30 wt. %aqueous solution of potassium hydroxide as the electrolyte.

[0060] (8) Charge-discharge cycle test

[0061] At room temperature, each test cell was charged at 50 mA/g for 8hours, then held at rest for 1 hour, subsequently discharged at 50 mA/gto a final discharge voltage of 0.9 V and thereafter held at rest for 1hour. This charge-discharge cycle was repeated, and the dischargecapacity (mAh/g) was measured every cycle.

[0062] In the case of the alkaline batteries, each battery was chargedwith current of 0.2 C for 6 hours and thereafter discharged at currentof 0.2 C to 1.0 V at room temperature to determine the initial dischargecapacity (discharge capacity of the first cycle).

[0063] (9) Measurement of electric resistance value

[0064] Each hydrogen absorbing alloy powder was checked for electricresistance value under the conditions of mean particle size of 35micrometers, pressure of 350 kgf/cm², test jig inside diameter of 12 mmand powder weight of 5 g.

[0065] (10) Test results

[0066]FIGS. 7 and 8 shows the results of the test.

[0067]FIG. 7 shows the initial discharge capacity (discharge capacity 1)of each test cell, and the initial discharge capacity (dischargecapacity 2) of each alkaline battery. Alloy electrodes A1 to A6 arethose prepared from the hydrogen absorbing alloys subjected to thereduction treatment with high-temperature hydrogen at 300° C. Alloyelectrodes B1 to B6 are those obtained from the hydrogen absorbingalloys not treated for reduction. Alloy electrodes A1 to A6 which aretreated for reduction are 285 mAh/g to 299 mAh/g in discharge capacity 1and 820 mAh to 865 mAh in discharge capacity 2, whereas alloy electrodesB1 to B6 which are not treated for reduction are 170 mAh/g to 246 mAh/gin discharge capacity 1 and 580 mAh to 675 mAh in discharge capacity 2.Thus, the alloy electrodes are greater in both discharge capacities andmore highly activated initially when treated for reduction thanotherwise.

[0068] Alloy electrodes A1 to A6 treated for reduction include thoseacid-treated or alkali-treated after the reduction treatment, and thosetreated neither with acid nor with alkali after the reduction.Acid-treated alloy electrodes A2 and A5 are 295 mAh/g and 299 mAh/g,respectively, in discharge capacity 1, and 860 mAh and 865 mAh,respectively, in discharge capacity 2. Alkali-treated alloy electrodesA3 and A6 are 290 mAh/g and 296 mAh/g, respectively, in dischargecapacity 1, and are both 855 mAh in discharge capacity 2. On the otherhand, untreated alloy electrodes A1 and A4 are 285 mAh/g and 292 mAh/g,respectively, in discharge capacity 1, and 820 mAh and 840 mAh,respectively, in discharge capacity 2. Thus, the acid-treated oralkali-treated alloy electrodes are greater in both discharge capacities1 and 2.

[0069] Accordingly, although the reduction treatment withhigh-temperature hydrogen, even when singly conducted, results in greatdischarge capacities as described above, further enhanced effects areavailable when the acid treatment or alkali treatment, preferably acidtreatment, is carried out after the reduction treatment.

[0070]FIG. 7 reveals that the alloy electrode of the invention (A2 orA3) subjected to the reduction treatment and the acid treatment is givenhigher activity than the conventional alloy electrode (B2 or B3) whichis subjected to the acid treatment only.

[0071] Alloy electrodes A1 to A3 and B1 to B3 are prepared from thealloy powder which is obtained by mechanically pulverizing an ingot madeby an argon arc furnace, while alloy electrodes A4 to A6 and B4 to B6are prepared from the alloy powder obtained by the gas atomizingprocess. These two groups of electrodes will be compared in dischargecapacities 1 and 2, as divided in two cases depending on whether thealloy is treated for reduction with high-temperature hydrogen orotherwise. In the case where no reduction treatment is conducted, alloyelectrodes B1 to B3 are 222 mAh/g to 246 mAh/g in discharge capacity 1and 620 mAh to 675 mAh in discharge capacity 2, while alloy electrodesB4 to B6 are 170 mAh/g to 221 mAh/g in discharge capacity 1 and 580 mAhto 620 mAh in discharge capacity 2. Thus, alloy electrodes B1 to B3prepared with use of the argon arc furnace are greater in both dischargecapacities 1 and 2.

[0072] In the case where the reduction treatment is conducted, on theother hand, alloy electrodes A1 to A3 are 285 mAh/g to 295 mAh/g indischarge capacity 1 and 820 mAh to 860 mAh in discharge capacity 2,while alloy electrodes A4 to A6 are 292 mAh/g to 299 mAh/g in dischargecapacity 1 and 840 mAh to 865 mAh in discharge capacity 2. Thus, alloyelectrodes A4 to A6 prepared by the gas atomizing process are greater inboth discharge capacities 1 and 2.

[0073] Accordingly, it is advantageous to prepare the alloy powder bythe gas atomizing process in respect of the initial activity in the casewhere the reduction treatment is conducted with high-temperaturehydrogen.

[0074] Furthermore, alloy electrodes A1 to A6 treated for reduction arelower than alloy electrodes B1 to B6 not treated for reduction in theresistance value of the powder as measured under the conditions ofparticle size of 35 micrometers, pressure of 350 kgf/cm², measuring jiginside diameter of 12 mm and powder weight of 5 g. This substantiatesthat the alloy particles are formed in their surface layer portions witha metal-rich layer having higher nickel and cobalt contents than in thealloy electrode not treated for reduction with high-temperaturehydrogen.

[0075]FIG. 8 shows the results obtained by measuring dischargecapacities 1 and 2 of alloy electrodes which were prepared from thealloy powder made with use of an argon arc furnace and the alloy powderobtained by the gas atomizing process after subjecting the powders tothe reduction treatment with high-temperature hydrogen at varyingtemperatures of 50° C. to 950° C.

[0076] The alloy electrodes prepared with use of the argon arc furnaceare as great as at least 284 mAh/g in discharge capacity 1 and at least820 mAh/g in discharge capacity 2 when the temperature for the reductiontreatment is in the range of 100° C. to 900° C.

[0077] The alloy electrodes prepared by the gas atomizing process are asgreat as at least 288 mAh/g in discharge capacity 1 and at least 835mAh/g in discharge capacity 2 when the temperature for the reductiontreatment is in the range of 100° C. to 900° C.

[0078] For the alloys thus used for testing, it is suitable that thetemperature for the reduction treatment be in the range of 100° C. to900° C. under the conditions of pressure of 1.2 atm and time period of30 minutes regardless of whether the argon arc furnace or the gasatomizing process is used. If the reduction treatment temperature is atleast 100° C., the equilibrium hydrogen pressure of the hydrogenabsorbing alloy rises to suppress the absorption of hydrogen by thealloy.

INDUSTRIAL APPLICABILITY

[0079] The hydrogen absorbing alloy power embodying the invention issuitable as a material for electrodes of metallic oxide-hydrogenbatteries, for example, as a material for the negative electrodes ofnickel-hydrogen batteries.

1. A hydrogen absorbing alloy powder containing nickel and cobalt, thepowder being characterized in that the alloy particles (1) are formedeach in a surface layer portion thereof with a metal-rich layer (3)enriched in metals by a reduction treatment with hydrogen, themetal-rich layer (3) being surface-treated with an acid or alkalinetreating liquid.
 2. A hydrogen absorbing alloy powder according to claim1 wherein the acid treating liquid is a hydrochloric acid solution.
 3. Ahydrogen absorbing alloy powder according to claim 1 wherein thealkaline treating liquid is at least one aqueous solution selected fromamong an aqueous solution of KOH, aqueous solution of NaOH and aqueoussolution of LiOH.
 4. A process for producing a hydrogen absorbing alloypowder characterized in that a starting hydrogen absorbing alloy powdercontaining nickel and cobalt is held in a high-temperature hydrogenatmosphere under the conditions of temperature, pressure and timesufficient to reduce oxides formed in a surface layer portion of each ofthe alloy particles (1), not melting the alloy particles (1) and notpermitting the alloy particles to absorb hydrogen, and thereaftersurface-treated with an acid or alkaline treating liquid.
 5. A processfor producing a hydrogen absorbing alloy powder according to claim 4wherein the temperature is in the range of 100° C. to 900° C.
 6. Aprocess for producing a hydrogen absorbing alloy powder according toclaim 4 or 5 wherein the pressure is in the range of 1 atm to 3 atm. 7.A process for producing a hydrogen absorbing alloy powder according toany one of claims 4 to 6 wherein the time is in the range of 30 minutesto 10 hours.
 8. A process for producing a hydrogen absorbing alloypowder according to any one of claims 4 to 7 wherein the startinghydrogen absorbing alloy powder is prepared by the gas atomizingprocess.
 9. A process for producing a hydrogen absorbing alloy powderaccording to any one of claims 4 to 8 wherein the acid treating liquidis a hydrochloric acid solution.
 10. A process for producing a hydrogenabsorbing alloy powder according to any one of claims 4 to 8 wherein thealkaline treating liquid is at least one aqueous solution selected fromamong an aqueous solution of KOH, aqueous solution of NaOH and aqueoussolution of LiOH.
 11. A hydrogen absorbing alloy electrode comprising anelectrically conductive substrate having applied thereto a hydrogenabsorbing alloy powder, the electrode being characterized in that thehydrogen absorbing alloy powder contains nickel and cobalt, the alloyparticles (1) being formed each in a surface layer portion thereof witha metal-rich layer (3) enriched in metals by a reduction treatment withhydrogen, the metal-rich layer (3) being surface-treated with an acid oralkaline treating liquid.
 12. A hydrogen absorbing alloy electrodeaccording to claim 11 wherein the acid treating liquid is a hydrochloricacid solution.
 13. A hydrogen absorbing alloy electrode according toclaim 11 wherein the alkaline treating liquid is at least one aqueoussolution selected from among an aqueous solution of KOH, aqueoussolution of NaOH and aqueous solution of LiOH.
 14. A metallicoxide-hydrogen battery wherein a hydrogen absorbing alloy electrode isused which comprises an electrically conductive substrate having appliedthereto a hydrogen absorbing alloy powder, the metallic oxide-hydrogenbattery being characterized in that the hydrogen absorbing alloy powdercontains nickel and cobalt, the alloy particles (1) being formed each ina surface layer portion thereof with a metal-rich layer (3) enriched inmetals by a reduction treatment with hydrogen, the metal-rich layer (3)being surface-treated with an acid or alkaline treating liquid.
 15. Ametallic oxide-hydrogen battery according to claim 14 wherein the acidtreating liquid is a hydrochloric acid solution.
 16. A metallicoxide-hydrogen battery according to claim 14 wherein the alkalinetreating liquid is at least one aqueous solution selected from among anaqueous solution of KOH, aqueous solution of NaOH and aqueous solutionof LiOH.